Respective Curie Points Research Articles

Structural and electrical properties of SrBi2V x Nb2−x O9 ferroelectric ceramics: Effect of temperature and frequency

Aurivillius type bismuth layered materials have received a lot of attention because of their application in ferroelectric non-volatile random access memories. Among bismuth layer structured ferroelectric ceramics SrBi2Ta2O9 (SBT)/SrBi2Nb2O9 (SBN) are of great interest for researchers because of their fatigue resistance and less distorted structure. Recently vanadium substitution in SBN/SBT has shown interesting electric and dielectric properties. In the present work, processing conditions, microstructure and electrical studies of vanadium doped SBN ferroelectric ceramics have been performed. Samples of compositions SrBi2V x Nb2-x O9, x = 0.0, 0.1, 0.3, 0.5 were prepared by solid-state reaction technique using high purity oxides / carbonates. The samples were calcined at 700 °C and sintered at 800 °C. X-ray diffractograms show that a single phase layered perovskite structure is formed in all the samples. Effect of partial substitution of pentavalent niobium ion (0.68 A) by smaller pentavalent vanadium ion (0.59 A) at B site on the microstructure, Curie temperature, Dielectric constant, Dielectric loss and electrical conductivity have been investigated. Dielectric properties of SBVN have been investigated from room temperature to 500 °C and frequency of 100 Hz to 1 MHz. Dielectric constant values at their respective Curie points are observed to increase with increasing vanadium concentration. Curie temperature is observed to be maximum in x = 0.1 vanadium doped sample. Strong relaxor like dielectric relaxation at the transition temperatures have been observed. With increasing vanadium concentration the dielectric loss is observed to increase significantly. It is also observed that dielectric loss increases with increase in temperature. The variation of conductivities in these samples is also reported.

Processing and Properties of Strontium Bismuth Vanadate Niobate Ferroelectric Ceramics

The processing conditions, microstructure, and dielectric properties of strontium bismuth niobate vanadate ceramics, SrBi2(VxNb1−x)2O9 (SBVN, with 0 ≤x≤ 0.3), were systematically studied. A relative density of >90% was obtained for all the samples using a two‐step sintering process. XRD showed that a single phase with the layered perovskite structure of SrBi2Nb2O9 (SBN) was formed with a vanadium content of up to 30 at.%. SEM revealed that the average grain size decreased gradually with an increase in vanadium content. The Curie point was found to gradually increase from ∼418°C for SBN to ∼459°C for SBVN with 30 at.% vanadium. Dielectric constants at room temperature and their respective Curie points were found to peak at a composition with 10–15 at.%; vanadium. Moreover, a high concentration of vanadium (>5 at.%) resulted in a significant increase in tangent loss at low frequencies (<1000 Hz). The relationships between chemical composition, processing condition, microstructure, and dielectric properties of SBVN ferroelectric ceramics are discussed.

The mechanism of self-reversal of thermoremanence in natural hemoilmenite crystals: new experimental data and model

New magnetic and mineralogical findings on self-reversing hemoilmenite (Fe 2− y Ti y O 3) grains from Pinatubo lavas (1991 eruption) provide important clues regarding the acquisition process of reverse thermoremanent magnetization (rTRM) in this solid solution series. Magnetic force microscopy indicates the presence of multidomain magnetic structures in coexisting strongly and weakly magnetic crystallographic regions having compositions of y≅0.54 and 0.53, respectively. Continuous thermal demagnetization of natural and laboratory TRM carried out on both whole rock samples and single hemoilmenite crystals shows that the magnitude of a normal TRM (nTRM) component, observed at temperatures above the Curie point of the self-reversing phase, is much too large to be carried by a phase that is entirely cation-disordered. Consistent with this observation are findings using transmission electron microscopy (TEM) which, in contrast to that what is commonly assumed, reveals the weakly magnetic regions to be magnetically heterogeneous. Specifically, these regions are found to contain tiny (20–40 nm) domains that are cation-ordered and evidently ferrimagnetic dispersed within the cation-disordered, presumably spin-canted antiferromagnetic matrix. Given these findings, we argue that the so-called nTRM-carrying x-phase is itself partially cation-ordered, and, thus, ferrimagnetic, as postulated first by Ishikawa and Syono (J. Phys. Soc., Jpn. 17 (1962) 714). We propose a “nanophase” self-reversal model for the ilmenite–hematite solid solution series in which the rTRM and nTRM components are carried by the cores and margins, respectively, of the tiny, partially cation-ordered nano-sized domains observed by TEM. Due to the partial cation order, both the core and the margin of each domain are expected to behave in a ferrimagnetic fashion at temperatures below their respective Curie points. However, given the kinetics of the ordering process, their cation distributions need be antiphase, which causes their magnetic moments to be oppositely aligned. Since it is most reasonable to consider each margin to be slightly more Fe-rich than the inside core, upon cooling, the margins acquire a magnetic remanence first (a nTRM). Then, upon further cooling, given that the intralayer and interlayer nearest-neighbor superexchange interactions are ferromagnetic and antiferromagnetic, respectively, the net magnetic moment of the core material need be oppositely aligned (producing a rTRM). The nano-sized regions would indeed behave in a superparamagnetic (SP) fashion if magnetically uncoupled to adjacent material; however, the spins in the margins (the x-phase) must be locked through superexchange to those of the surrounding disordered matrix, which we also claim to be locally enriched in iron. If so, then the magnetization of the x-phase can be both highly-coercive and thermally stable, as observed experimentally. Upon stepwise thermal demagnetization, the self-reversed remanence measured at room temperature is not destroyed until the unblocking temperature of the disordered Fe-enriched aureole (approximately 410°C) is reached. Mineralogical considerations and magnetic evidence from previous works suggest that this model is generally valid for self-reversed dacitic pumice, in particular the Mt. Haruna dacite and the 1985 Nevado del Ruiz dacitic andesite.

Enhanced Ferroelectric Properties of V-Doped BaBi4Ti4O15 Single Crystal

Single crystals of barium bismuth titanate (BaBi4Ti4O15) and BaBi4Ti4O15 partially substituted with V5+ and Ba2+ for Ti4+ and Bi3+ sites, respectively, at 5 at.% (Ba1.2Bi3.8Ti3.8V0.2O15), were grown using slow cooling. The incorporation of vanadium into the B-site of the layered pseudo-perovskite block resulted in a shift of the Curie point to a higher temperature, from 410°C to 425°C, and an increase in dielectric permittivity from 8000 to 9500 along the a(b)-axis direction and from 150 to 160 along the c-axis direction at 1 MHz and at their respective Curie points. The increase in Curie temperature brought about an increase in remanent polarization, and saturated remanent polarization was enhanced from 14.8 µC cm-2 to 15.5 µC cm-2.

Ferroelectric and Dielectric Properties of Strontium Bismuth Niobate Vanadates

Strontium bismuth niobate vanadates, SrBi2 (VxNb1-x)2O9 (with 0 ≤ x ≤ 0.1), were prepared by reaction sintering of powder mixtures of constituent oxides. With partial substitution of niobium by vanadium cations (up to 10 at.%), the single-phase layered perovskite structure was preserved, and the sintering temperature of the system was significantly lowered (∼200 °C). The incorporation of vanadium into the layered perovskite structure resulted in a shift of the Curie point to higher temperatures from 435 to 457 °C, with 10 at.% vanadium doping, and an increase in dielectric constant from ∼700 to ∼1100, with 10 at.% vanadium doping, at their respective Curie points. The remanent polarization increased from ∼2.4 to ∼8 µC/cm2, while the coercive field decreased from ∼63 to ∼45 kV/cm with 10 at.% V5+ doping.

Enhanced ferroelectric properties and lowered processing temperatures of strontium bismuth niobates with vanadium doping

This letter reports on enhanced ferroelectric properties and lowered processing temperatures of a SrBi2(V, Nb)2O9 system with vanadium doping. The samples were prepared by reaction sintering of powder mixtures of constituent oxides. With partial substitution of niobium by vanadium cations (up to 10 at. %), the single-phase layered perovskite structure was preserved and the sintering temperature of the system was significantly lowered (∼ 200 °C). The incorporation of vanadium into the layered perovskite structure resulted in a shift of the Curie point to higher temperatures, from 435 to 457 °C with 10 at. % vanadium doping, and an increase in dielectric constant, from ∼700 to ∼1200 with 5 at. % vanadium doping, at their respective Curie points. Both enhanced Curie temperatures and dielectric constants at the Curie points indicate an increase in polarizability, which could be attributed to the increased “rattling space” due to the incorporation of much smaller vanadium cations. Further measurements also revealed that loss tangent of vanadium doped layered perovskites was the same as that without vanadium doping.

Magnetic properties of Tb/sub 1-x/Nd/sub x/Cd

Magnetic moment measurements on a series of Tb/sub 1-x/Nd/sub x/Cd pseudobinary alloys have been carried out from helium temperatures to room temperatures and in magnetic fields up to 1.4 MA/m. The Curie temperatures are observed to initially increase as Nd is substituted for Tb, with a maximum at x/spl cong/0.25. Ferromagnetic coupling of the Tb-Nd spins results in antiparallel Tb (heavy rare earth) and Nd (light rare earth) magnetic moments throughout the series, with magnetization cancellation at x/spl cong/0.75. Sharp maxima in the magnetizations at temperatures below their respective Curie points are interpreted as marking magnetization reorientation temperatures.

Magnetic properties of the ordered VPt3 alloy. - II. — Critical behaviour around the Curie point

The ferromagnetic properties of the ordered VPt 3 alloy have been systematically investigated in a previous paper (Paper I). These investigations were carried out on bulk VPt 3 samples, one ordered in the Cu 3 Au structure (the VPt 3 (1) sample) and the other ordered in the TiAl 3 structure (the VPt 3 (2) sample); disordered (Cu like) VPt 3 is only paramagnetic. In the present paper, we report a detailed investigation on the critical properties of VPt 3 (1) and (2) around their respective Curie points (240 K and 210 K). The corresponding magnetization measurements were performed in the moderate field range 0.5 kOe≤H≤20 kOe, at several temperatures ranging from 195 K to 271 K for VPt 3 (1) and from 188 K to 235 K for VPt 3 (2). The iterative method used for the determination of T e , β, γ and δ is described in detail. We found T e =240.0 K, β=0.50, γ=1.19 and δ=3.38 for VPt 3 (1), against T c =210.3 K, β=0.51, γ=1.21 and δ=3.38 for VPt 3 (2). The critical properties of both VPt 3 samples are well described in the frame of the static scaling theory: Widom's scaling law γ=β(δ−1) is well satisfied and the critical DHW (Domb, Hunter, Widom) equation of state holds for VPt 3 (1) and (2). All results obtained here (critical exponents and amplitudes, critical magnetic equation of state), are discussed and compared with theory. We compare the critical magnetic equation of state of VPt 3 (1) and (2) with that of the most appro Etude detaillee du comportement critique des phases paramagnetique desordonnee (type Cu) et ferromagnetique ordonnee (type TiAl 3 ) de l'alliage VPt 3 autour des points de Curie respectifs (240 K, 210 K). Mesures d'aimantation et determination de la temperature critique et des exposants critiques. La loi d'echelle est verifiee. La transition magnetique est bien representee par l'equation critique d'etat magnetique. Comparaison aux modeles theoriques. Un modele d'interactions a longue portee convient pour les deux phases

Hopkinson effect, blocking temperature and Curie point in basalts

A critical study of the thermal decay of TRM as well as the thermal variation of low field susceptibility of several basalts has been made. The most important results emerging from this study are (i) for some basalts there is a large difference between the maximum blocking temperature of the grains and their Curie points and (ii) some show susceptibility peaks far below their respective Curie points whereby this phenomenon could be considered to be different from the usual Hopkinson effect. A variety of susceptibility versus temperature curves have been obtained for basalts which could be explained in terms of three basic types of curves.

Optical Study of X-Ray Irradiation Effect on the Phase Transition in Triglycine Sulfate and Diglycine Nitrate

Infrared spectra are measured at various temperatures on ferroelectric triglycine sulfate (Tc=48°C) and diglycine nitrate (Tc=−67°C) and nonferroelectric diglycine hydrochloride, each of which includes a short hydrogen bond connecting two glycine molecules. Almost all of the spectra of these substances are only slightly changed as the temperature passes through the respective Curie points. In contrast, a band at 575 cm−1 in triglycine sulfate is very characteristically changed, and its behavior is strongly influenced by x-ray irradiation. Further, a detailed inspection of the spectra shows that above and below the respective Curie points one of the two glycine molecules is in zwitterion form in diglycine nitrate and not in triglycine sulfate, and that the potential for the proton is of symmetrical single-minimum type in the latter and of asymmetrical double-minimum type in the former. Based on these facts, it is suggested that the origin of the phase transition, in triglycine sulfate, is not attributable to the hydrogen bond but to the remarkable intensity behavior of the 575-cm−1 band. On the other hand, in diglycine nitrate it is ascribed to a collective motion leading to the interchanging of the hydrogens between two possible equilibrium positions on the hydrogen bonds.

Nuclear Magnetic Resonance and X-Ray Studies of Potassium Ferrocyanide Trihydrate Crystal

The atomic coordinates of one previously unknown water oxygen in potassium ferrocyanide trihydrate are determined by X-ray structure analysis in the paraelectric phase at room temperature. In K 4 Fe(CN) 6 ·3H 2 O and its isomorphous K 4 Ru(CN) 6 ·3H 2 O proton magnetic resonance patterns start to narrow at about -140°C, and definite change in both spectra occurs in the same temperature region around -40°C, about twenty degrees below the respective Curie points. Angular dependence of peak separations has been measured at liquid nitrogen temperature where all water molecules are effectively rigid. Direction angles of three proton-proton vectors with respect to the crystallographic a , b and c axes are, respectively, α 0 =134°, β 0 =49°, γ 0 =73°; α 0 =90°, β 0 =90°, γ 0 =0° and α 0 =0°, β 0 =90°, γ 0 =90°. A possible molecular mechanism for the ferroelectric origin is discussed.

The Variation with Temperature of the Spectral Emissivities of Iron, Nickel and Cobalt

The spectral emissivities of iron, nickel and cobalt have been measured over a wavelength range 1.0? to 3.0? and at temperatures between 650? and 1350?C. The results show anomalies in the cases of iron and cobalt occurring at temperatures in the neighbourhood of the respective Curie points. Tables of the temperature coefficients of emissivity for the three metals are given.

The Measurement of Magnetic Hysteresis in Rocks and Minerals at High Temperatures

An experimental method is described, designed chiefly to furnish data for a study of the direction, and stability with time, of thermoremanence in rocks. Specimens were heated to the Curie point in an evacuated electric furnace. Two pick-up coils were arranged close to the gap of a tuned a. c. electromagnet providing a maximum field H of 2400 oersteds. These were balanced to make their resultant e. m. f. zero in the presence of H alone, and proportional to dI/dt when a specimen of intensity of magnetization I was in the gap. This e. m. f. was applied to the vertical plates of a cathode ray oscilloscope. The potential drop over a small resistance in the electromagnet input was applied to the deflection coils, giving a measure of H. Computations based on the resulting pattern on the c. r. o. screen yield a loop of the I-H type, with I in arbitrary units, from which the coercivity can be evaluated. A pronounced “sawtooth” pattern has been observed in the (dI/dt)-H traces of pyrrhotite and franklinite specimens, particularly just below the respective Curie points.

Structure and thermal properties associated with some hydrogen bonds in crystals VII. Behaviour of KH 2 PO 4 and KH 2 A sO 4 on cooling

A low-temperature micro-thermostat is described, suitable for single-crystal X-ray work. Thermal expansions of KH 2 PO 4 (and in less detail of KH 2 AsO 4 ) have been measured by X-ray methods from room temperatures down to about 80° K. Both these crystals differ from some previously investigated organic compounds with short hydrogen bonds, in that maximum thermal contraction on cooling occurs along the c axis, which is approximately perpendicular to the planes containing the hydrogen bonds in these two crystals. This axis becomes electrically polarized below a thermal ‘transition point’. Remarkable effects are observed in the neighbourhood of the transition point, similar to those previously described for Rochelle salt. When the tetragonal single crystals are cooled below the respective Curie points (123° K for KH 2 PO 4 and 96° K for KH 2 AsO 4 ) they change into crystal ‘hybrids’, in which subcrystalline units of lower symmetry pack so as to preserve the c axis and either the (100) or (010) planes of the original tetragonal structure, and to maintain electrical neutrality on the average. When a ‘hybrid ’ crystal is warmed above the Curie point, the subcrystalline units merge after a varying lapse of time into the original tetragonal lattice. This change from single crystal to hybrid and back can be repeated at will by cooling and heating. The structure of these crystal hybrids has been investigated by both Bragg and Laue methods. On the basis of the new experimental evidence and of previous work, an explanation is offered for phase transitions ‘of the second kind’, preferably termed ‘continuous’, in terms of crystal structure. It is known that ‘discontinuous’ phase changes at a transition ‘point’ involve a rearrangement of molecules within the single crystal of one structure, to give a powder of crystals with a different structure. But if the new crystals can pack as sub­crystalline units into the original crystal lattice, as is possible when the difference in structure is small and not too much strain is involved, the phase change will occur continuously over a range of temperatures instead of discontinuously at a transition point. The X-ray studies described give new information about the thermodynamics of crystals, and promise to throw light on certain problems of imperfect crystals. Possible biological analogues of hybrid crystals are briefly referred to.